With the emergence of new services such as Video On Demand (VOD), high definition television, and online gaming, the bandwidth required by users is increasing. The development of Fiber To The Home (FTTH) ensures the last mile bandwidth of the access network. The PON technology is one of the most widely applied FTTH technologies currently.
FIG. 1 shows a network architecture of a traditional PON in the conventional art. As shown in FIG. 1, in which a PON system is composed of an Optical Line Terminal (OLT), an Optical Network Unit (ONU), an optical splitter, and an Optical Distribution Network (ODN). The OLT is connected to the optical splitter through a trunk fiber, and the ONU is connected to the optical splitter through a branch fiber. The trunk fiber, optical splitter, and branch fiber constitute an ODN. The direction from the OLT to the ONU is the downstream direction, and the direction from the ONU to the OLT is the upstream direction.
The conventional art provides a PON system protection model to avoid PON system failures. FIG. 2 shows a Gigabit-PON (GPON) system backup model in the conventional art. In the backup model shown in FIG. 2, both the ONU and the OLT have two ODN interfaces in addition to the ODN backup. This protection model may have four protection modes. FIG. 3a is a type-A protection mode, which provides backup only for the trunk fiber between the OLT and the optical splitter. FIG. 3b shows a type-B protection mode, which provides backup for both the trunk fiber and the OLT. FIG. 3c shows a type-C protection mode, which provides backup for the OLT, the trunk fiber, the optical splitter, the branch fibers, and all ONUs. FIG. 3d shows a type-D protection mode, which provides backup for the OLT, the trunk fiber, the optical splitter, the branch fibers, and some ONUs. Table 1 shows the characteristics of the four protection modes. In practice, a proper protection mode may be selected according to the actual requirements and the characteristics of the four protection modes.
TABLE 1WhetherFramesor SignalsProtectionBackupAre Lost inProtectionModeRedundant DeviceStateSwitchingCostType ATwo fibers, a singleColdYesLowOLT, a single ONU, andbackupa single optical splitterType BTwo fibers, two OLTs, aColdYesModeratesingle ONU, and a singlebackupoptical splitterType CTwo fibers, two OLTs,HotNoHighesttwo ONUs, and twobackupoptical splittersType DTwo fibers, two OLTs,ColdYesHighpartially two ONUs, andbackuptwo groups of opticalsplitter pairs
In a traditional PON system, few ONUs are connected to the OLT through the optical splitter, and the coverage radius is small. Consequently, in the traditional network structure, the quantity of OLTs is large, and the location areas are remote and distributed, thus bringing inconvenience of management and maintenance. With the emergence of the next-generation optical access network, the LR-PON system is proposed. Because the all optical long-reach technology is difficult to implement and costly, the solution characterised by Electrical Relay (E-R)-based long-reach regenerator comes forth. FIG. 4 shows a structure of an LR-PON system implemented through an E-R device in the conventional art. As shown in FIG. 4, an E-R device is set between the optical splitter and the OLT, and a traditional ODN is divided into two ODNs, namely, ODN 1 and OND 2 in FIG. 4. The E-R device regenerates the signal, namely, performs Reamplifying, Reshaping, Retiming (3R) for the signal, thus reducing the signal defect as a result of long reach of the OLT in an LR-PON system in the transmission. FIG. 5 shows a structure of an E-R device in the conventional art. As shown in FIG. 5, the E-R device includes an optical-to-electrical converting unit (O/E unit) 501, a signal regenerating unit 502, and an electrical-to-optical converting unit (E/O unit) 503. The O/E unit 501 is configured to receive the downstream optical signal from the OLT, or receive the upstream optical signal from the ONU, and convert the received optical signal into an electrical signal. The signal regenerating unit 502 is configured to perform 3R processing for the converted electrical signal from the O/E unit 501. The E/O unit 503 is configured to convert the electrical signal processed by the signal regenerating unit 502 into an optical signal, and send the signal.
An active E-R device is introduced into the LR-PON, thus increasing instability of the LR-PON system. The optical transmission path is extended from 20 km to 100 km, and the failure probability of the long-reach fiber path is increased. After the coverage is widened, more services are affected once a failure occurs. Therefore, the LR-PON system needs protection urgently. However, no protection method is available for protecting the LR-PON system at present.